The present invention relates in general to pull-up bolting assemblies used as mine roof or rock bolt anchoring mechanisms, and more particularly to mine roof or rock bolt anchor mechanisms of the resin anchored type adapted to be introduced into a bore hole and having a leading end portion adapted to be anchored in the bore hole by a quick setting adhesive after which rotation of the threaded bolt portion draws the bolt portion upwardly to achieve compression force in the rock mass.
In the past, two principal approaches have been used in providing anchor bolts for strengthening roofs in a coal mine, a subway tunnel or similar subterranean structures or to strengthen a rock mass. One type of construction employed parts that are expanded upon insertion within a drill or bore hole to provide a mechanical type of expansion gripping action, permitting a bolt or screw to be tightened upwardly against a face of the rock structure to achieve strengthening of the rock mass. The bolting assembly was required to enable a tension pull-up or tightening down of the bolt and bearing plate assembly in compliance with Bureau of Mines Standards established for the industry, including requirements that such an anchor bolt unit be capable of achieving installed bolt tensions within the range of at least 6,000 to 8,000 pounds.
A more recent type of assembly involves the use of adhesive or grouting material within the bore hole or drill hole for securing the rod or bolt in place, and if pull-up tensioning was required, the outer end of the rod or bolt was threaded and provided with a nut and faceplate washer, permitting the plate to be pulled up against the front face of the rock mass or mine roof when the nut was tightened.
The mechanical type of anchor bolt described above has been somewhat limited in regard to the conditions under which it an be used, as it requires a relatively strong or more stable type of stratum and has not proven satisfactory for many types of formations encountered, particularly where mudstone or siltstone, limestone and laminated sandstone or shale formations occur. The second or adhesive type of anchor bolt unit has the advantage that the adhesive may be employed to strengthen the particular rock formation as well as secure the bolt in position in the bore hole, but such an adhesive type unit has been rather expensive and has required great amounts of adhesive when applied in accordance with the customary practice of substantially filling the hole about the full length of the rod or bolt member.
There has been a general recognition of the need for improved types of bolting assemblies which will make possible the achievement of greater amounts of face compression and draw up tension strength, and which will also reduce expense from the standpoint of installation time and labor and materials.
In an effort to provide a bolting unit of the resin type which would achieve improved compression strength and draw up tension strength, and seek to reduce expense of material and labor and time involved in installation, a type of resin anchored bolt assembly has been proposed in earlier U.S. Pat. No. 3,877,235 wherein a threaded anchor unit was provided with a hollow cylindrical leading portion resembling a pipe-like shell, which was assembled to an anchor bolt and provided with a restraining device that permitted the threaded leading end cylindrical anchor portion to rotate with the anchor bolt, without relative rotation between them, whereby the shell-like anchor portion may be inserted into the bore hole behind a capsule or cartridge of adhesive resin and employed to crush and mix the resin, whereby, when the resin has cured to adhesively secure the anchor portion in the bore hole, application of a predetermined amount of torque to the bolt cause the restraining device to release the bolt for rotation relative to the anchor device and tighten it to generate compressive force in the rock mass. This was achieved by providing a fracturable disc member forming a bottom wall of the pipe-like hollow cylindrical anchor portion to which a standard nut was welded, so that when the anchor portion was held by the resin against further rotation in the bore hole, sufficient torque could be applied to the bolt to fracture and penetrate the frangible disc and allow the bolt upon tightening to be drawn up into the bore hole to produce the desired compressive force effects.
An object of the present invention is the provision of an improved mine roof or rock bolt anchor mechanism of the resin anchored type, wherein the leading end anchor portion of the assembly is readily formed of commercially available material of economic composition of a readily available type, and wherein a nut member with threads deformed or mismatched relative to the threads of the bolt is fixed to the anchor portion and resists rotation of the anchor portion relative to the bolt until sufficient restraint of anchor portion rotation by resin cure occurs, whereupon application of torque to the bolt exceeds the prevailing torque of the nut and bolt threads, whereupon the bolt can be drawn up to tightened condition in the rock mass.
Another object of the present invention is the provision of a threaded anchor and bolt assembly of the resin anchored type, wherein an anchor portion formed of a generally cylindrical body of readily commercially available material is provided with a hollow bore and a nut having threads so related to threads of the bolt as to form a restraining device having prevailing torque characteristics similar to those of metal lock nuts, arranged to prevent or restrain relative rotation of the anchor portion with respect to the bolt upon rotation of the bolt during insertion and resin mixing phases of installation, and wherein the prevailing torque is exceeded by applied torque after the resin is set and restrains the anchor portion against rotation to allow tightening of the bolt to apply the desired compressive force to the rock mass.
Other objects, advantages and capabilities of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings illustrating preferred embodiments of the invention.